1. Field of the Invention
The present invention relates to magnetic method of reproducing a signal recorded in a magnetic recording medium, and an apparatus for recording a signal in a magnetic recording medium and reproducing the signal therefrom.
2. Discussion of Background
A conventional magnetic recording and reproducing apparatus has main components of a magnetic recording medium for receiving a signal and an induction-type magnetic head for recording the signal to the medium and reproducing the signal therefrom. However, as the recording density of magnetic recording apparatus was increased, the magnitude of magnetic field of the signal generated by a single recording bit was decreased, whereby it became difficult to obtain S/N ratio of a sufficiently large magnitude by the use of the induction-type magnetic head. For this, magnetroresistive (MR) head was developed in place of the induction-type magnetic head and already proceeded to be used practically in a hard disk device and so on. In this method, the same principle as that of the conventional head was used, but the reproducing was performed by a MR head having large sensitivity of detecting magnetic field. However, when the recording density was increased using a MR head, a decrease of S/N ratio was inevitable. Therefore, there were proposed improved technologies, namely a giant magnetroresistive (GMR) head of which magnetic film was substituted by a multi-layer magnetic film having a giant magnetroresistive effect and a tunneling magnetroresistive (TMR) head of which magnetic film was substituted by a tunnel junction multi-layer film having a spin tunnel effect. FIG. 5 shows a basic structure of the TMR head in the conventional magnetic recording and reproducing apparatus, for example shown in "Nikkei Electronics", Volume 1997/4/7, pages 125 through 129 (reference 1). In the Figure, numeral 1 designates an upper magnetic pole for recording; numeral 2 designates a lower magnetic pole for recording which also works as an upper shield for reproducing; numeral 3 designates a recording coil; numeral 4 designate a reproducing electrodes; numeral 5 designates a magnetic recording medium; numeral 6 designates a TMR multi-layer film; and numeral 7 designates a lower shield for reproducing.
Before explaining about the operation of the present invention, a spin tunnel effect, which is the most important element of the TMR-head, is explained. FIGS. 6(a) and 6(b) are cross-sectional views of the TMR (tunneling magnetroresistive) multi-layer film, wherein numeral 8 designates a magnetic film (pin layer); numeral 9 designates a magnetic film (free layer); numeral 10 designates an insulating layer; and numeral 11 designates an antiferromagnetic film. Specifically, FIG. 6 (a) shows a case that the TMR multi-layer film has a high resistance. FIG. 6 (b) shows a case that the TMR multi-layer film has a low resistance. When a voltage is applied between the magnetic film 8 (pin layer) and the magnetic film 9 (free layer), and the thickness of insulating layer 10 is about several .mu.m, a tunnel current flows through these magnetic films. Provided that at least one of the magnetic films is a non-magnetic substance, an ordinary tunnel effect is observed. However, both of the magnetic films are a magnetic substance, the tunnel current becomes large when the direction of magnetization of the magnetic film 8 (pin layer) and that of the magnetic film 9 (free layer) are the same and the tunnel current becomes small when the directions of magnetization are reverse to each other because the tunnel current has the tunnel effect of electrons having a spin direction corresponding to the magnetization of the magnetic film. Accordingly, when the direction of magnetization of the magnetic film 8 (pin layer) is fixed by a magnetic coupling with the antiferromagnetic film 11 and an alternating magnetic field is applied to the whole multi-layer film, only the direction of magnetization of the magnetic film 9 (free layer) is reversed, thereby the magnitude of tunnel current changes and the magnetic field signal becomes detectable. According to a recent report, the maximum rate of the change in the tunnel resistance is 24% under the room temperature. The spin tunnel effect appears in the same manner even though the clearance between the magnetic films opposing to each other is a vacuum or a gas instead of an insulant.
Next, operation of the TMR head is described. In FIG. 5, when a signal of-electric current is applied to a recording coil 3, a magnetic field corresponding to the signal is generated in a gap between an upper magnetic pole for recording 1 and a lower magnetic pole for recording 2. Depending on the magnetic field, a signal is recorded in a magnetic recording medium 5 as the direction and the magnitude of magnetization. The above operation is common both to the conventional technique and to improved technique. The reproducing is performed by detecting the signal of magnetic field generated from the magnetic recording medium 5 by the TMR multi-layer film 6. In other words, when the TMR head is driven relatively to the magnetic recording medium over the magnetic recording medium with an application of a voltage between the reproducing electrodes, the signal of magnetic field applied to the TMR multi-layer film 6 is changed. Therefore, the direction of magnetization of the magnetic film 9(free layer) is changed in response thereto. Thus, the electric current flows between the reproducing electrodes are changed. The upper shield for reproducing 2 and the lower shield for reproducing 7 are a magnetic shield for improving a resolution power by allowing to pick up only the signal of magnetic field lies just under the TMR multi-layer film 6.
On the other hand, there was proposed a method called magnetic force microscope which utilizes a magnetic force between a magnetic recording medium and a needle-like magnetic head located closely to the magnetic recording medium with a distance of about dozens of nm. For example, according to a thesis by P. C. D. Hobbs, D. W. Abraham and H. K. Wickramasinghe (reference 2) described in journal "Appl. Phys. Lett.", 55(1989) pages 2357 through 2359, there was disclosed that a pattern of recording magnetization could be read out at a resolution power of 25 .mu.m by measuring vertical displacement of a needle-like magnetic head caused by a magnetic force using an beam interferometer. Further, according to a thesis by J. Moreland, and P. Rice (reference 3) in journal "Appl. Phys. Lett." 57(1990) pages 310 through 312, there was disclosed that a recording magnetization pattern was read at a resolution power of 20 nm by measuring a tunnel current in a gap between a magnetic recording medium and a needle-like magnetic head in order to obtain vertical displacement. However, the latter utilized a change of the magnitude of tunnel current caused in accordance with the width of the gap, wherein the spin tunnel effect was not utilized.
Meanwhile, in a conventional horizontal magnetic recording medium as shown in FIG. 4, the stability of recorded bits of the magnetic recording medium was lost as recording density thereof increased. For this, a vertical magnetic recording medium which can maintain recorded bits stably even under a high recording density is being developed. In the vertical magnetic recording medium, the direction of magnetization of recorded signals was perpendicular to the film surface, therefore it was necessary to use a recording film having a tendency to magnetize in the direction vertical to the film surface. For example, in a thesis by Y. Sonobe, Y. Ikeda, H. Uchida, and T. Toyooka (reference 4) described in journal "IEEE Tran, Mag." 32(1996), pages 3801 through 3804, there was disclosed that a recording and reproducing width of 2.5 .mu.m and a magnetic flux reversal density of 5.9 kFR/mm (corresponding to a resolution power of 170 .mu.m) were experimentally obtained using a recording medium made of alloy of Co, Pt, Cr and Ta as the vertical magnetic recording medium and a MR head having a structure that the TMR multi-layer film of the TMR head shown in FIG. 4 was substituted by an ordinal MR film as the magnetic head.
Although, in the conventional MR head, it was possible to obtain such a recording density by combining the vertical magnetic recording medium, there was a difficult problem in order to increase the recording density. For example, in order to obtain a recording and reproducing width of as much as a half of 1.1 .mu.m, it was necessary to halve the width of the MR film in the track width direction. In this case, it was also necessary to keep the magnetizing direction of the MR head in an optimum direction to halve the height of the MR film in how order to maintain the value of electric resistance the same. For example, in a thesis by C. Tsang, H. Santini, D. MacCown, J. Lo, and R. Lee (reference 5) described in journal "IEEE Tran. Mag," 32(1996), pages 7 through 12, there was disclosed that the maximum recording density is attained by the conventional technic, namely, the height of the MR head was a minute value of 0.5 .mu.m despite the width of the MR head is 1.3 .mu.m, and the height was obtained by a mechanical abrasion of the head. Thus, it was difficult to realize industrially a narrow track width of 1 .mu.m or less by the conventional method. Such a problem in manufacturing was not solvable using a GMR head or a TMR head even though a reproducing sensitivity can be improved. Further, although there was a measure of thinning the film thickness of the MR film or the GMR film in order to maintain a requisite value of electric resistance, it was not an efficient means for solving the problem because the characteristics of the MR effect and the GMR effect are generally deteriorated when the film thickness was reduced.
Meanwhile, according to the method used in a magnetic force microscope, the resolution power can largely be improved. In the conventional magnetic force microscope, a mechanical displacement of the needle-like magnetic head caused by a magnetic force was detected optically or electrically. However, in order to realize such a detection, it was necessary to reduce the rigidity of structure including the magnetic head as much as it can respond to variation of the magnetic force. On the other hand, when the rigidity was excessively small, a gap between the magnetic head and the recording medium could not be kept against a disturbance, and also the structure, the size and the manufacturing process of the magnetic head were largely restricted. Therefore, the rate of responding to a signal was dominated by the rigidity of the magnetic head, thereby the reproducing was possible only for frequency of several kHz. Thus it was difficult to obtain a reproductive signal having a frequency of several MHz or more, which is the standard frequency of magnetic disk devices at present.
Further, means for recording a signal in the vertical magnetic recording medium at a high density can be realized by, for example, reducing the gap width of the conventional head, using the needle-like magnetic head, or using a laser beam, wherein problems along with high density recording mainly occur in reproducing means.